1. Field of the Invention
The present invention relates to a film forming composition comprising a resin and modified nanoparticles. The nanoparticles are modified to improve surface migration of the nanoparticles in a film. The invention also relates to novel modified nanoparticles and to the use of modified nanoparticles in film forming compositions, in particular in clear coating compositions, for improving surface properties, in particular scratch resistance.
2. Description of the Related Art
WO 03/102089 describes nanoparticles modified with compounds that lower the surface tension. The nanoparticles are enriched at the surface of the coating. The particles are chemically modified, leading to a non-reversible covalent chemical bond, by using silane coupling agents such as the octyltriethoxysilane, perfluorooctyltriethoxysilane and reaction product of e.g. aminopropyltriethoxysilane and epoxy eicosane. Such covalent chemical treatments, however, are based on expensive silane chemicals. Moreover, during the chemical modification not all silane material is attached to the particles but remains in part—also as oligomers—dissolved in the solution of the modified nanoparticles. Further, during the storage slow reaction takes place of these oligomeric species containing reactive groups such as silanol groups which causes storage stability problems such as loss of clarity and viscosity increase of the dispersion. Such aged dispersion is not any more suitable for use in coatings especially not in clear coatings. Another disadvantage is that during the film formation and curing both the surface active particles and the remaining surface active unattached silane material will go in competition in migrating to the surface of the coating. The lower molecular weight silane species migrate faster due to the higher mobility.
PCT/EP2006/061826 describes nanoparticles chemically modified with a cross-linked polysiloxane. The particles are surface active yielding enrichment of the particles at the surface of the cured film-forming resin. The enrichment is shown with TEM (i.e. Transmission Electron Micrographs) and/or ToF-SIMS (i.e. Time-of-Flight Secondary Ion Mass Spectroscopy). The disadvantage of this modification is that it is relatively expensive.
PCT/US2005/018656 describes film forming compositions comprising crystalline nanoparticles such as alumina powder dispersed by means of a polymeric dispersant. The polymeric dispersant helps to disperse the nanoparticles to their primary particles size and prevents agglomeration. Upon extra addition of surface active chemicals, for example silicone or fluor types these dispersed nanoparticles migrate to the surface. The surface activity of the dispersed nanoparticles is said to be caused by the interaction of the surface active chemicals with the polymeric dispersant. This enables the migration of the nanoparticles to the surface of the coating and results in improved scratch resistance. A drawback of such approach is that it is cumbersome, critical and expensive due to the use of both dispersant and surface active agent. It is difficult to prevent that such surface active agent interferes with wetting or levelling agents that are usually added to a film forming composition for other purposes, for example to get a good film appearance.
There is still a need for a nanoparticle based additive for film-forming compositions, in particular coating compositions that enhances the scratch resistance of coatings and has less or none of the above described disadvantages of the prior art modified nanoparticles and that is simple to use and is inexpensive.